Approximately 350,000 deaths occur each year in the United States, due to sudden cardiac arrest (SCA). Many of these deaths can be prevented if effective defibrillation is administered within 3-5 minutes of SCA.
Sudden cardiac arrest is the onset of an abnormal heart rhythm, lack of pulse and absence of breath, leading to a loss of consciousness. If a pulse is not restored within a few minutes, death occurs. Most often, SCA is due to ventricular fibrillation, a chaotic heart rhythm that causes an uncoordinated quivering of the heart muscle. The lack of coordinated heart muscle contractions results in insufficient blood flow to the brain and other organs. Unless this chaotic rhythm is terminated, allowing the heart to restore its own normal rhythm and thus normal blood flow to the brain and other organs, death ensues.
Rapid defibrillation is the only known means to restore the normal heart rhythm and prevent death after SCA due to ventricular fibrillation. For each minute that passes after the onset of SCA, the mortality rate increases by 10%. If defibrillated within 1-2 minutes, a patient's survival rate can be as high as 90% or more. At 7-10 minutes, the patient's survival rate drops below 10%. Therefore, the only way to increase the survival chances for an SCA victim is through early defibrillation.
Automatic External Defibrillators (AEDs) can provide early access to defibrillation, but they must be portable so they can be easily carried to a victim of SCA, easy-to-use so that they can be properly utilized when SCA occurs, and easily maintained. In addition, AEDs must be inexpensive, so that they can be broadly deployed.
Additionally, AEDs require a portable energy source to enable the device to be deployed quickly to treat a victim of SCA. Often, the victim may be in a remote or difficult-to-reach area, making compact and portable AEDs attractive to police, EMT, Search-And-Rescue and other rescue or emergency services.
AEDs must remain in a standby mode for extended periods of time. Most current AEDs are rated for two years of standby and must be able to complete a sufficient number of shocks at the end of this period. However, during this two-year standby period, the battery pack may discharge significantly and thus may not have sufficient capacity to provide therapy, especially in situations which may require many defibrillation shocks and an extended period of monitoring time.
Currently, many AEDs use a battery monitoring circuit, also known as a “smart battery”, to provide a “fuel gauge” for remaining capacity. This technique requires the use of low power analog and digital circuitry within the battery pack or the device to constantly monitor battery capacity. Most of these devices also monitor battery temperature in order to accurately gauge capacity. As those skilled in the art can appreciate, the disadvantage of this technique is that the additional circuitry, components and connections needed to monitor battery capacity may add significant cost to the battery pack and/or the AED itself. As is well known to those skilled in the art, this technique has been historically problematic and has been an issue with portable AEDs that use either disposable or rechargeable battery packs.